In general, the compounding of a rubber composition for a tire remarkably depends on raw materials derived from petroleum resources. For example, a general tire for a passenger car on the market today contains about 20% of synthetic rubber, about 20% of carbon black, a softener, synthetic fiber and the like with respect to the total weight of the tire. In other words, at least 50% of the overall tire is constituted of raw materials derived from petroleum resources.
For example, synthetic rubber such as butadiene rubber (BR), carbon black etc. have been employed for a rubber composition for the tread of the tire, in order to improve gripping performance and abrasion resistance. Carbon black directed to low rolling resistance (LRR) has been blended into a rubber composition for the base tread of the tire, in addition to natural rubber (NR) exhibiting excellent rolling resistance. While the rigidity of bead apex rubber must be improved in order to obtain a tire having high steering stability, a large quantity of carbon black has generally been blended into a rubber composition for the bead apex, in order to improve the rigidity thereof.
Synthetic rubber and carbon black have been blended also into clinch rubber, in order to obtain a tire exhibiting excellent steering stability and extensibility in traveling in a well-balanced manner.
Further, butyl rubber, halogenated butyl rubber or the like having excellent air permeation resistance has been employed for inner liner rubber, and carbon black has also been blended into the same in order to improve flex cracking resistance. Both of the synthetic rubber such as butyl rubber or halogenated butyl rubber and carbon black are derived from petroleum resources.
In recent years, however, a special emphasis has been put on environmental issues, and the emission of carbon dioxide has been strictly limited. Further, the reserves of petroleum are limited, and hence raw materials derived from petroleum resources cannot be unlimitedly used. Such environmental awareness also affects the field of tires, to result in a demand for a rubber composition for a tire prepared by partially or entirely substituting raw materials derived from nonpetroleum resources for the currently used raw materials derived from petroleum resources. The substitutional raw materials derived from nonpetroleum resources must maintain characteristics at least equivalent to or superior to those of the raw materials derived from petroleum resources.
Japanese Patent Laying-Open No. 2003-63206 (Patent Document 1) discloses a rubber composition for a tread employing natural rubber or epoxidized natural rubber, silica etc. as raw materials derived from nonpetroleum resources for reducing dependency on petroleum resources. In this rubber composition for a tread, however, no abrasion resistance is taken into consideration, and the gripping performance thereof is still insufficient.
Japanese Patent Laying-Open No. 2003-55505 (Patent Document 2) discloses a rubber composition containing 0.1 to 20 parts by weight of zinc oxide having a specific surface area of at least 10 m2/g and preferably having an average particle diameter of at least 1 μm with respect to 100 parts by weight of a diene rubber component. However, this rubber composition is still insufficient in gripping performance and abrasion resistance.
Thus, no rubber composition having excellent gripping performance and abrasion resistance for serving as the material for a tread has been obtained with raw materials derived from nonpetroleum resources substituting for those derived from petroleum resources.
While a base tread of a tire must have excellent rolling resistance and high-speed durability, no such properties are taken into consideration in relation to the rubber composition disclosed in the aforementioned Patent Document 1.
Japanese Patent Laying-Open No. 2003-12866 (Patent Document 3) discloses a rubber composition for a base tread containing a rubber component, carbon black having a specific iodine adsorption, silica and sulfur. However, no high-speed durability is taken into consideration in relation to this rubber composition either.
Thus, a rubber composition having excellent low rolling resistance (LRR) and high-speed durability sufficiently applicable to base tread rubber is not yet known in relation to a rubber composition prepared by substituting raw materials derived from nonpetroleum resources for those derived from petroleum resources.
Japanese Patent Laying-Open No. 2002-37929 (Patent Document 4) discloses a rubber composition for a bead apex reinforced with used paper such as old newspaper in consideration of saving of resources and environmental protection. However, the rubber composition described in Patent Document 4 contains a large quantity of carbon black derived from petroleum resources as a filler for reinforcement along with the old paper, and saving of resources and environmental protection are not sufficiently taken into consideration. While the rigidity of the bead apex can be improved due to the large quantity of carbon black, the tire easily generates heat during traveling, and hence the durability of the tire is deteriorated due to reduction of dynamic fatigability. Further, the rolling resistance of the tire is disadvantageously increased due to increase of the loss tangent (tan δ).
Japanese Patent Laying-Open No. 2005-247984 (Patent Document 5) discloses a rubber composition for a clinch containing silica-containing carbon black and carbon black. However, the rubber composition for a clinch having different properties cannot be applied to a bead apex as such, while tensile strength and flex cracking resistance are not taken into consideration in this rubber composition.
A clinch rubber member is arranged on a region in contact with a rim over a sidewall and a bead of a tire, and positioned on a portion chafing against the rim when the tire is mounted on the rim. The clinch rubber member of the tire must have high hardness and high heat aging resistance in order to transmit driving force from the rim to the tire and to hold the load on the tire in traveling. Further, the clinch rubber member must have high abrasion resistance, in order to prevent abrasion resulting from chafing against the rim following repeated deformation of the tire in traveling. In addition, the rigidity, the hardness and the strength of the clinch rubber member remarkably influence the steering stability in traveling.
The clinch rubber member must also exhibit high breaking extension (extensibility), in order to prevent the toe of the tire from chipping frequently caused when the tire is exchanged.
Japanese Patent Laying-Open No. 7-118444 (1995: Patent Document 6) proposes a hard rubber composition employing polybutadiene rubber containing at least 5 percent by weight of syndiotactic crystals as a rubber composition employed for the clinch rubber member required to have excellent properties. Japanese Patent Laying-Open No. 7-81335 (1995: Patent Document 7) discloses a chafer rubber member consisting of a composite rubber strip prepared by bonding several types of rubber components having different blending ratios and characteristics to each other. In order to prepare a multilayered clinch or chafer rubber member, however, the manufacturing steps are complicated, to result in a high cost.
Japanese Patent Laying-Open No. 2000-198883 (Patent Document 8) discloses a rubber composition for a tire tread compatibly attaining low rolling resistance and high abrasion resistance by blending a specific quantity of a carbon black material containing silica. If this technique is applied to a clinch rubber member required to have high rigidity and high hardness, however, steering stability and extensibility cannot be improved in a well-balanced manner. The aforementioned Patent Document 5 describes the rubber composition for a clinch containing silica-containing carbon black and carbon black. However, this rubber composition is still insufficient in tensile strength and abrasion resistance.
An inner liner is a rubber member forming the inner surface of a pneumatic tire, particularly a tubeless tire, for retaining the internal pressure of the tire. While the inner liner rubber member must have excellent air permeation resistance and flex cracking resistance, neither air permeation resistance nor flex cracking resistance is taken into consideration in relation to the aforementioned rubber composition according to Patent Document 1.
Japanese Patent Laying-Open No. 2005-264114 (Patent Document 9) discloses a rubber composition prepared by dispersing a specific quantity of zinc oxide into a rubber component containing halogenated butyl rubber and/or a halide of a copolymer of isobutylene and p-methylstyrene and kneading the same an inner liner consisting of this rubber composition, and describes that the rubber gauge of the inner liner or the like can be kept thin and the weight of the tire can be reduced without reducing the viscosity. However, no technique of compatibly attaining air permeation resistance and flex cracking resistance of the inner liner rubber is taken into consideration.
Thus, a rubber composition having excellent air permeation resistance and flex cracking resistance sufficiently employable as the material for an inner liner is not yet known in relation to a rubber composition prepared from raw materials derived from nonpetroleum resources substituting for those derived from petroleum resources.    Patent Document 1: Japanese Patent Laying-Open No. 2003-63206    Patent Document 2: Japanese Patent Laying-Open No. 2003-55505    Patent Document 3: Japanese Patent Laying-Open No. 2003-12866    Patent Document 4: Japanese Patent Laying-Open No. 2002-37929    Patent Document 5: Japanese Patent Laying-Open No. 2005-247984    Patent Document 6: Japanese Patent Laying-Open No. 7-118444    Patent Document 7: Japanese Patent Laying-Open No. 7-81335    Patent Document 8: Japanese Patent Laying-Open No. 2000-198883    Patent Document 9: Japanese Patent Laying-Open No. 2005-264114